Abstract
Purpose:
Bitemporal hemianopia (BTH) is a heteronymous visual field (VF) loss in both eyes due to lesions at the optic chiasm. Previous field expansion devices for BTH required scanning into the prism and the prism apical scotoma created a blind area. Thus, a novel multiplexing prism (MxP) was developed that expands VFs without apical scotoma. We optimized the MxP design to maximize the size of BTH patient’s expanded VF and developed prototype glasses for bilateral nasal field expansion.
Methods:
The residual nasal field on each side is limited to ≈55°. We placed two MxPs (base-in) over the nose bridge to expand the nasal VF bilaterally. The MxPs prevent apical scotomas by superimposing both see-through and shifted views. The MxP configuration for wide field expansion was determined by considering the rated and effective prism powers, and interference between both prisms. Increasing the rated prism power expands VF farther, while rotating the prism nasal edge towards the eye can increase the effective prism power due to the increased angle of incidence. The deflected ray at the nasal edge of one prism has to pass forward to the temporal edge of the fellow eye’s prism, constraining the amount of rotation.
Results:
Field expansion for BTH using MxPs is limited to 90° nasally by the nose and interference with the fellow eye’s prism. Ray tracing shows that even with moderate (30Δ) rated prism power (with MxP serration directed towards the eye) effective prism power of 70Δ can be achieved by rotating the prism 10°. On the other hand, high power prism (57Δ) requires a much high rotation angle (30°) to achieve same effective prism power without the interference by fellow eye’s prism. The rotation results in minification of the shifted image by the induced variation of prism power. The expanded VF is compressed and projected into a smaller part of the patient’s VF, effectively expanding both nasal VFs. Prototype MxP glasses for BTH were implemented with 40Δ MxPs rotated by 15°. Wrap-around single lens glasses were used to provide space for the rotated prisms. The binocular VF of about 110° in the BTH patient was expanded up to 80° in each eye giving a binocular VF of about 160°.
Conclusions:
A novel design of MxPs mounted nasally in both eyes can expand VF to compensate for the temporal crescents loss. The rotation angle, the rated power of prism, and the size of prism are best fitted individually due to variations in facial biometry.
Keywords: 584 low vision •
642 perimetry •
611 neuro-ophthalmology: cortical function/rehabilitation